Techniques of applying a photosensitive resin composition comprising a polymeric binder, an ethylenically unsaturated monomer, and a triazine photopolymerization initiator to making printing plates, forming a photoresist, and the like are hitherto known. Triazine photopolymerization initiators are widely employed because of their higher sensitivity to thereby achieve satisfactory image reproducibility, as compared to various other initiators, such as benzophenone initiators, anthraquinone initiators and thioxanthone initiators.
Examples of known photosensitive resin compositions as described above include a photosensitive image-forming composition comprising a 2,4,5-triarylimidazolyl dimer as a photopolymerization initiator and an addition polymerizable ethylenically unsaturated monomer as disclosed in JP-A-60-202437 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and a photosensitive composition containing a photosensitive s-triazine compound as disclosed in JP-A-60-239736.
The triazine compounds described in these publications have a disadvantage of poor storage stability and are apt to produce scum during development. In order to overcome this problem, triazine compounds having a specific structure have been proposed as disclosed in JP-A-63-68831 and JP-A-63-70243. In recent years, however, there has been a demand for the development of photosensitive compositions having a higher resolving power and improved developability in the field of printing plates, printed circuit boards, electronic devices, and the like, and even the above-mentioned improved triazine compounds are still unsatisfactory in terms of both resolving power and developability.
With the latest technological development of electronic devices such as computers, factory automation has also advanced in the field of printing, enabling integral processing from the input of an original or image data through plate making, inclusive of editing and proofreading. In such an advanced plate making technique, direct imaging using a laser beam is employed. The laser beam thus used is preferably an argon laser beam having a wavelength of 488 nm in the visible region. Hence, there has been a demand to develop a resin composition having photosensitivity to visible light. Various photosensitive resin compositions meeting this demand have been proposed to date as described, e.g., in JP-A-63-260909, JP-A-1-105238, JP-A-1-203413, JP-A-1-203414, JP-A-2-1714, JP-A-2-73813, JP-A-2-127404, and JP-A-3-239703.
Of these proposals, the compositions described in JP-A-63-260909, JP-A-1-105238, JP-A-1-203413, JP-A-203414, JP-A-2-1714 and JP-A-2-73813 use an acridine compound and a trihalomethyl-containing triazine compound as a photopolymerization initiator, and the compositions of JP-A-2-127404 and JP-A-3-239703 use a metallocene compound in addition to the above-mentioned photopolymerization initiators.
On the other hand, with respect to a presensitized plate (hereinafter abbreviated as a PS plate) suitable for producing a litho printing plate, a known process for producing a PS plate comprises subjecting a surface-grained aluminum plate 1 to anodizing to form oxide layer 2, rendering the oxide layer hydrophilic, and then coating a photosensitive resin composition on the resulting aluminum substrate to form photosensitive resin layer 3 as shown in FIG. 1. If desired, overcoat layer 4 may be provided on the resin layer as an oxygen barrier.
When the above-mentioned PS plate, etc. is exposed to high energy radiation, such as ultraviolet light, a film original must be used, which gives rise to a problem of poor image reproducibility due to the poor image reproduction of the film itself and insufficient contact between the film and the photosensitive resin layer. When a laser beam is used for exposure, image formation is conducted by direct scanning without using a film original and is therefore free from the problem of reproducibility. The problem associated with image formation with a laser beam is that a high-energy short-wavelength laser has a short life for practical use and that a laser having a relatively long wavelength, such as an argon laser, has low energy and demands a highly sensitive photosensitive resin composition. There has therefore been a demand to develop such a highly sensitive photosensitive resin composition.
Returning to the compositions using an acridine compound and a trihalomethyl-containing triazine compound, their sensitivity to argon laser light is insufficient. There are also problems which still remain unresolved in that a long exposure time is required and there is a reduction in image reproducibility due to insufficient curing. The technique using a metallocene compound in combination as disclosed in JP-A-2-127404 also fails to satisfy all the requirements of sensitivity, adhesion, developability and pattern forming precision.
On the other hand, photosensitive resin compositions have been applied, for example, to color filters for liquid crystal display devices and pre-sensitized plates (PS plates) for use in lithography.
An example of a method for producing a color filter is the pigment dispersion method which permits easy processing operations. This pigment dispersion method comprises dispersing a pigment to be incorporated into a photosensitive resin composition, applying the photosensitive resin composition to a substrate, drying, then exposing through a pattern mask and developing to thereby form a relief resin layer having a desired color. A PS plate for use in lithography is typically prepared by surface-graining an aluminum substrate, anodizing the substrate to form an oxide coating film, rendering the surface of the oxide film hydrophilic, and then applying a photosensitive resin composition onto this oxide coating film to thereby form a photosensitive resin layer.
However, conventional photosensitive resin compositions employed in the production of color filters by the pigment dispersion method are insufficient in the sensitivity that can be achieved by exposure through a pattern mask. Hence, exposure time can hardly be shortened and a high-energy light source is required for exposure. Namely, these conventional photosensitive resin compositions fail to meet the demand for saving energy. Conventional photosensitive resin compositions are also disadvantageous in that they have poor resolution and fine-line reproducibility, and thus it is difficult to produce high-quality color filters using such resin compositions. Furthermore, the conventional photosensitive resin compositions have another problem in that they provide for small development margins (e.g., an optimum time range for achieving satisfactory developing process). As a result, even exposed areas will come off the substrate if the processing time exceeds the optimum time no matter how small the excess is.
In the case of a PS plate, a photosensitive resin layer is generally exposed, for example, to high-energy radiation such as UV light through a pattern mask and then developed. However, there have been several problems including poor image production of the pattern mask per se and poor image reproducibility due to insufficient contact between the pattern mask and the photosensitive resin layer. These problems have been solved to a level that is satisfactory for the time being. This is because the recently developed direct drawing method which allows patterns to be directly drawn by a laser beam scanning exposure has made it possible to perform integral processing from input of an original or image data to platemaking, inclusive of editing and proofreading and processing. Thus, pattern masks are no longer required. However, when this direct drawing method is effected using laser beams having a relatively long wavelength and low energy (for example, an argon laser having a wavelength of 488 nm in the visible region), a photosensitive resin composition must be used which not only has high sensitivity to low-energy visible light, but which also assures high resolution. In fact, however, photosensitive resin compositions which possess these characteristics so far have not been discovered.